In the past, various photosensitive resin compositions or various heat curable resin compositions have been used for forming a solder mask layer on a printed wiring board.
Recently, electronic devices have been required to be lighter, thinner, shorter and smaller, and therefore printed wiring boards has been required to have wires in a higher density. To meet these requirements, the solder mask layer is required to have high performance. That is, a resin composition for a solder mask layer is required to be less likely to gel and excellent in developability, and can be formed into an excellent solder mask layer having sufficient properties such as hardness, heat resistance, chemical resistance, electrical insulation reliability, flexibility, and toughness.
In view of the above circumstances, the applicants proposed a curable composition capable of being developed by alkaline developers as disclosed in Patent Literature 1. The curable composition contains: (A) a carboxyl-containing resin which has an acid value ranging from 20 to 200 mg KOH/g and is soluble to an organic solvent; (B) a carboxyl-containing compound; (C) a photosensitive (meth)acrylate compound; and (D) a photopolymerization initiator. The carboxyl-containing resin is prepared by reacting epoxy groups of a resin (a) which contains two or more epoxy groups per one molecule with 0.3 to 0.9 mol of one type or more of a monocarboxylic acid (b) in total per one equivalent of the epoxy groups of the resin to prepare a reaction product (c), and subsequently reacting epoxy groups of the reaction product (c) with 1.0 to 5.0 mol of a polybasic acid (d) per one equivalent of the epoxy groups of the reaction product. According to the invention disclosed in Patent Literature 1, there is provided the curable composition having sufficient developability for increasing a density of wires in a printed wiring board. Furthermore, the curable composition can form a cured film excellent in hardness, solder heat resistance, chemical resistance, adhesion, PCT resistance, electroless gold plating resistance, whitening resistance, electrical insulation reliability, and flexibility.
However, the composition described in Patent Literature 1 may gel at the time of curing, unfortunately, depending on a selection of raw materials and reaction conditions. Furthermore, in view of recent requirements to increase a density of wires in a printed wiring board, the solder mask layer is requested to have further improved electrical insulation reliability.